Pyridine nucleotide interaction with rat liver dihydropteridine reductase.

The interactions of a homogeneous preparation of rat liver dihydropteridine reductase with NADH, NADPH, NAD+, NADP+, and the 1-N6-ethenoadenine derivative of NAD+ have been investigated by fluorescence titration, circular dichroism, equilibrium dialysis, Sephadex G-25 chromatography, and polyacrylamide gel electrophoresis. The procedures indicate that the dimeric enzyme has a definite preference for NADH, but binds only 1 mol of this nucleotide per mol of enzyme. The binary complex of enzyme with NADH is only partially stable to exhaustive dialysis and gel electrophoresis, where it shows greater mobility (0.26) than the free enzyme (0.21); however, the complex can be isolated by Sephadex G-25 chromatography, and characterized with respect to its absorbance spectrum. No ternary complexes are observed when samples of reductase, preincubated with excess NADH, and either the reaction product, 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine, or the inhibitor, methotrexate, are subjected to polyacrylamide gel electrophoresis.